U.S. Pat. No. 5,409,047 discloses a chipper knife assembly, in which a reversible knife is used. In order to support the reversible knife, the knife assembly includes a clamp and a counter knife, between which the reversible knife is pressed. In a known manner, there are two opposite cutting bevel edges, making it possible to set the reversible knife in the knife assembly in two different ways. The material of the reversible knife can thus be utilized as efficiently as possible. In the said patent, an attempt has been made to improve efficiency by arranging the reversible knife so that it can be sharpened. For this purpose, a key, as a locking piece, is arranged between the counter knife and the reversible knife, to prevent the lateral movement of the reversible knife relative to the counter knife. In other words, the key holds the reversible knife in place against the shear force. The reversible knife has a groove for the key. Correspondingly, there are several parallel groves in the counter knife for the key, allowing the position of the reversible knife in relation to the knife frame to be set as desired. The longitudinal grooves in the counter knife for the reversible knife form an adjustment tolerance, within which the reversible knife can be moved. Once there is no longer any sharpening tolerance or aforesaid adjustment tolerance in the reversible knife, the reversible knife is replaced with a new one.
The key used in the disclosed knife assembly is especially thin and thus liable to break. In addition, the key is difficult to position correctly and when fitting the reversible knife the clamp too must be removed. This is because the grooves in the counter knife only cover part of the width of the reversible knife. At the same time, several keys are required for a single reversible knife, making it even more difficult to set the reversible knife. In practice, the counter knife too wears during chipping. However, the disclosed knife assembly uses one and the same counter knife for a long time. In addition, the changing of the counter knife is labourious, as it is attached with screws. Further, the load arising in operation tends to lift the reversible knife and the counter knife. Sawdust then becomes packed between them, bending the knives and reducing the conducting of heat away from the knives. In addition, a most of the load acting on the reversible knife and the counter knife is directed onto the screws that act as the securing means.